This revised R01 application, proposed by a new investigator, requests 3 years of support to study gender differences in the epidemiology of MDMA (Ecstasy) and other hallucinogen use. We propose to conduct secondary data analyses of existing data: the 2001-2004 National Surveys on Drug Use and Health (NSDUH) and the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC). We will examine four specific aims. The first aim is to examine gender differences in the incidence and determinants of first use of MDMA and other hallucinogens, including incidence trends, risk periods for initiation, and correlates of recent initiation. The second aim is to determine gender differences in the pattern, sequence, and correlates of multi-drug use among hallucinogen users, including injection drug use (IDU) and use of methamphetamine, cocaine/crack, and heroin. The third aim is to examine gender differences in the natural history of hallucinogen use disorders and correlates of hallucinogen use disorders, including the progression from first use to disorder onset, to onsets of other substance use disorders, and temporal associations with mental disorders. The fourth aim is to investigate gender differences in the prevalence, patterns, and correlates of substance abuse treatment service use among hallucinogen users, including the location of services received, the primary abused substance for receiving treatment, and perceived unmet needs for and barriers to treatment use. Our proposed analysis will help identify the heightened risk periods for initiation and demographic subgroups at risk for using MDMA and other hallucinogens to facilitate targeting primary prevention programs. The findings also will specify subgroups of MDMA/hallucinogen users who have an increased likelihood of IDU, multidrug use, drug abuse or dependence, and other mental disorders, so they can be targeted for focused programs or interventions to reduce serious consequences of chronic MDMA and other drug abuse. We will understand better the pattern of, unmet needs for, and barriers to substance abuse service use among MDMA/hallucinogen users. Given the extremely high cost of a sophisticated longitudinal study, our proposed secondary analysis of already collected data is an initial and cost-effective step to study the distributions, correlates, courses, consequences, and treatment use of hallucinogen users.